The effects of ionizing and ultra-violet radiation on nucleic acids and proteins and their constituents are being studied. The modification of radiation damage in DNA by cancer chemotherapy agents of the intercalating and alkylating types is of interest, since such information may be useful in radiation therapy. The present report includes electron spin resonance (e.s.r.) studies of spintrapped free radicals produced by Gamma-radiolysis in the polycrystalline state and in aqueous solutions of amino acids, peptides and of nucleic acid constituents. Radicals were also generated by ultra violet photolysis, dibenzoyl peroxide photoinduced reactions and ultrasound. Conclusive evidence for the formation of hydroxyl radicals and of hydrogen atoms in cavitation bubbles during sonolysis of aqueous solutions has been obtained by spin trapping and e.s.r.